Hyperacusis and noise sensitivity are types of decreased sound tolerance (DST) disorders, and frequently reported in patients with tinnitus and head injury respectively. When any degree of DST is seen in patients, it is paramount to address the problems tolerating sounds because it negatively impacts every aspect of daily functioning; severe DST manifests as everyday sounds being perceived as too loud, overwhelming, and annoying. Due to the paucity of research on DST, limited knowledge exists pertaining to fundamental aspects of its pathophysiology and how best to treat it. The overall objectives of this application are to test two conceptual models of DST, an auditory model (Aim 1) and a psychological model (Aim 2) to determine the etiology of severe DST. When Veterans are seen in the clinic with complaints of sound intolerance (i.e., severe DST), health care providers are uncertain how to help their patients. There are no objective tests to diagnose DST and the commonly used behavioral assessment of loudness perception can result in exacerbating DST symptoms and emotional distress. Additionally, the standard test battery does not provide any useful information as to the underlying nature of the problem. This proposal is the first step to achieve our long-term goals of understanding the etiology of DST and ultimately, provide clinicians the information they need to effectively treat these disorders. To accomplish our goals, it is critical to use a behavioral measure of loudness perception that does not cause unnecessary distress or exacerbate DST symptoms. Categorical Loudness Scaling (CLS) is an automated procedure used to quantify loudness judgments. Part of this proposal will be to refine the CLS procedure so that it meets this need. The design of this study is to test multiple stages of the conceptual models using physiological measures of auditory and psychological function. By examining the relationship between auditory and psychological biomarkers, this study will provide clinically relevant information on where deficits exist in sensory and/or neurological structures that lead to severe DST. This knowledge will guide future studies so that rehabilitative treatments can be developed targeting the underlying mechanism that leads to severe DST. This research is taking the necessary steps to provide an evidence-based approach to direct clinical decision-making for Veterans with severe DST. Outcomes of the proposed work have both theoretical and clinical impact. The experiments outlined in this proposal will elucidate the underlying pathophysiology that leads to severe DST and result in valid and efficient quantitative metrics that could become part of a standardized test battery for evaluating this health condition. Additionally, by identifying risk factors, including the influence of mental health on degree of DST, outcomes of this proposal will guide future studies on the development of a care path, one that will be interdisciplinary in its design, that can address the complexity of severe DST and provide health care professionals the resources they need to best meet the needs of their patients with any degree of DST.